[image: image.jpeg] On Sat, May 30, 2020 at 7:31 AM cogoman via Emc-users < emc-users@lists.sourceforge.net> wrote:
> > On 5/29/20 8:44 PM, Gene Heskett wrote: > > In this case, openscad gives me the tools I want right now, and I could > > even do a set of gears for stage 2 of the reduction since that spacing > > is almost too close for a belt coupling. So by using the correct tooth > > profiles it could make a very usefull gearset, no belt at all, and a > > higher reduction at the same time. But would those teeth be strong > > enough. Worth a try IMO. And a swarf cover from the printer should be > > doable. > Two points: 1) can you (you, not someone else) actually modify a downloaded gear design using openscad? Try it Go get any random gear from SPD/SI or Grainger and change the hub and face width and save as STL. I bet this is not easy. It is nearly trivial in Fusion or Onshape or any that work like them. Seriously, go through the steps using each system. Downloading and installing 3 or 4 CAD systems takes days but doing the same test project in each is the only way to see what each can do. Downloading a gear and making a trivial modification like (boring out the hub) then saving as STL is a good test case. 2) Yes, the TEETH would be strong enough to carry the force from any reasonable size motor. Remember that a 1NM motor can only generate 1NM of torque. What is the radius of the gear? lets guess at 0.1 meter. If so then the force n the tooth is 10 Newtons (or in barbarian units about 2.5 pounds) and this is in the worst case where the motor is stalled at full current against a hard stop. When you slice the gear you can specify the wall thickness and the infill percent and style. For gears of this size in PLA I'd go with 1.6mm walls and skin and 50% infill density. Cutting those numbers in half to 0.8 and 25% would still make a serviceable gear for a small < 2NM motor. PLA is the most rigid plastic. It is not the toughest against impacts but for driving a CNC machine rigidity matters more than impact resistance I've been doing experiments with plastic gears. I can print small ones down to module 0.5 but the percent error in tooth shape is very high for small gears. For my self-drive car project I am using module 2.0 spur gears with 24mm face width. These gears are huge but the front tire will be hitting rocks and the computer will be making fast steering corrections. These gears are very strong so that "something else" will break first. But notice I said the TEETH are very strong. In a gear the stres is greatest near the hub and minimal on the teeth. Larger diameter gears have less force on the teeth then do smaller diameter gears. So you always want to use the largest gears possible. If the gear fails, it fails at the hub. It slips on the shaft after the keywhy shears off of the threads holding the setscrew strip out. With PLA gear slipping on the hub is a "death spiral" because slipping make heat and heat softens the PLA and makes it slip more. And the hub to shaft interfce is the highest stressed place. It fails if not over-engineered. My solution is to make my plastic gears with a 24mm (or larger) diameter borehole. Then turn a 24mm steel bushing that fits the shaft and has set screws. he gear is press fit with epoxy to the bushing. This way the hub is steel and will never fail and the teeth never do fail. (Later I might try making hexagonal hub.) That said. When you design the gear, just figure that PLA s about 1/3rd as strong as metal and give the plastic gear a wider face and try to use module 1.0 size at least if you can. Finally one more thing: With plastic gears, you really can benefit from using *helical* gears as more tooth area is in contact and they slide better. It costs no more to print a helix than a spur gear. Next, you think you don't want to deal with axial loads (side force) so you use TWO helix gears to make herringbone gears. These have zero side force, are self-aligning, and cost no more than spur gears. MANY people doing gears with printers use this kind of gear. They are expensive to cut in steel but cost no more in plastic. If you have a CAD system herringbone gears are easy. (1) find a helix gear some place like Grainger or McMaster Carr, Download both the left and right-hand versions. (2) in the CAD system stick them together, side by side, (3) cut the face width as required, (3) bore the hub to 24mm, (4) slice and print. Try doing the full exercise using OnShape then Oopenscad then with fusion and see which works best for you. (If you are a hard-core Linux user I bet onshape works best) As a further exercise try getting you CAD system to make g-code for a 4-axis mill to make the same herringbone gears. Try using a tiny ball-end cutter. Here is a photo. [image: images.jpeg] > > > Strength of 3d printed parts is a deep subject. PLA if done right can > be pretty strong, but other filaments done right are stronger. > > I like this video showing how a veteran at 3d printing goes about > refining his process for printing in nylon. > > https://www.youtube.com/watch?v=5MFX1whwjGg > > Here is a video where some guys test out different filaments on a 15 > horsepower outboard motor. > > https://www.youtube.com/watch?v=scUEEQyC0GI > > > Keep in mind, the odor will be much worse using any filament besides > PLA, and most will need a hot end that can do the higher temperatures. > > Also with higher temperatures you might want to put a box around the > printer to keep drafts from messing up a print. One guy uses grow tents > around his printers. > > > > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California
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